Make skip_on_field_errors default to True?

[sloria]

As a follow-up to #346, we may make skip_on_field_errors default to True.

[immerrr]

I don't think it's good to omit a check that verifies that data['x'] < data['y'] if data['foobar'] is invalid, so I'd say I'd expect @ma.validates_schema validator to run by default. However I'd expect the validator to not run by default for @ma.validates('foobar') if foobar validation failed.

Thinking it deeper, it might be better to enable the user to specify the list of pre-requisite fields validated by a certain @ma.validates_schema-validator and don't run that validator if some of the pre-requisite fields failed to validate on their own. Even better would be to have an auto-detection mechanism for such fields, but none comes to mind at this moment. And it doesn't seem that much of a user chore to "declare interest" in certain fields by adding skip_on_field_errors=['x', 'y'] when writing the validator.